Select everything except ears from mesh of head
Another way: If you don't mind adding some extra points on the model, you may also consider to separate ears by cutting them off using Knife Cut or Knife Project tool:





PROS: Almost perfect clean separation.

CONS: Add extra vertices to the original mesh. You may judge and weigh if this is acceptable.

Then CtrlG to add those vertices to a new group for the restraining usage in Decimate modifier panel.